The present invention relates to semiconductor devices, and relates in particular to an improved method for making ohmic contacts of stable electrical characteristics to shallow semiconductor regions thereof.
In semiconductor devices, such as insulated gate field effect transistors, the source and drain electrodes usually comprise a low melting point metal such as aluminum, which has been deposited on the surface of the source and drain regions making ohmic contact therewith. While aluminum may be satisfactory for this purpose in such devices that have relatively deep source and drain regions, i.e. depths greatly exceeding one micron, it has not proven satisfactory for making ohmic contact to source and drain regions which are very shallow, such as 0.5 micron or less in depth. Specifically, in fabricating the metal electrode, the aluminum is first deposited on the surface of the source and drain regions, and then subjected to a heat treatment at a temperature ranging from 400.degree. C. to 500.degree. C. to ensure an electrical connection. During this heat treatment, aluminum diffuses through the shallow depth of the semiconductor regions and thereby increase leakage current, and finally cause the underlying semiconductor junction to be short circuited. This destruction of the semiconductor junction is termed in the art "junction spiking".
The connection of metal electrode to a semiconductor region is generally performed by etching away an insulating layer, which protects the surface of a semiconductor body or substrate and a PN semiconductor junction reaching the surface at a predetermined location to form a contact window. Electrode material, such as aluminum, is then deposited on the surface of the semiconductor region through the contact window. The formation of the contact window in the insulating layer is provided in accordance with well-known lithographic techniques. The formation of the semiconductor region in the semiconductor body is provided by using a diffusion mask having windows at predetermined locations, which are also formed in accordance with lithographic techniques. Ideally, it is preferred that each contact window and diffusion window be precisely formed at predetermined locations with a specified size. However, because some misalignment of the mask patterns for forming the contact window and diffusion window is unavoidable, the contact window frequently exposes a portion of the semiconductor body. As a result, in forming the interconnecting layer, shortcircuiting of the underlying semiconductor junction occurs irrespective of junction spiking. These problems which are associated with mask misalignment are inherent to the lithographic techniques, and because of these problems, an increase in density of integrated circuit elements becomes difficult.